Add wait events for server logging destination writes

Hi hackers,

The write(2) calls that flush server log output aren't covered by wait
events. When a backend logs something, the writes go out in:

- write_pipe_chunks(): write(2) to the syslogger pipe
- write_console(): write(2) to stderr (WriteConsoleW() on Windows)

If one of those blocks -- syslogger pipe full, slow console, slow log
device -- pg_stat_activity just shows wait_event = NULL until it
returns. Since NULL usually reads as "on CPU", a backend stuck writing
logs looks like it's doing work, so logging-related stalls are easy to
miss.

Attached is a short series that adds two WaitEventIO events and reports
them around those writes:

IO / SysloggerWrite - write(2) to the syslogger pipe
IO / StderrWrite - write(2) to stderr, and WriteConsoleW()

0001 adds the events and covers the write(2) paths. 0002 does the
Windows WriteConsoleW() path, split out since it's platform-specific.

It only wraps the leaf write call and uses the existing
pgstat_report_wait_start()/end() helpers, so it stays allocation-free
and safe to call from inside the error-reporting path.

I did a quick before/after to make sure the events show up: 8 backends
each emitting large RAISE LOG lines, sampling wait_event from
pg_stat_activity every 50 ms for 20 s.

- logging_collector = on (syslogger pipe):
master: NULL 100.0% (2184/2184)
patched: IO/SysloggerWrite 99.1% (2204/2224), NULL 0.9%

- logging_collector = off (stderr):
master: NULL 100.0% (2144/2144)
patched: IO/StderrWrite 90.7% (1952/2152), NULL 9.3%

On master that wait time is just invisible; with the patch it lands on
the new events. I can send the scripts and raw samples if anyone wants
to reproduce it.

Applies on current master. A couple of things I'm unsure about and
would appreciate input on: whether the event names fit the surrounding
conventions, and whether splitting the Windows path into its own patch
is the right call.

Thanks,
Seongjun Shin